Never seen a tank fail, regs.. oh yes. Visual inspection is a must with the fiber ones, my local field is pretty strict on checking the hydro dates. And knowing the abuse a tank can take on the field whether playing or being thrown down by a unhappy player due to marker malfunction or dare I say getting shot out by a noob
I think that at least a test every few years is a must. We are lucky enough to have a place in town that hydros them for under $15 so isn't to big a deal every couple of years. Does that test mean the tanks safe for those few years, nope. Take that fancy cover off and check it while cleaning your gear is you best bet like alpha says.